The present invention generally relates to radio frequency (RF) signal amplification, and particularly relates to a method and apparatus for efficient, linear RF signal amplification.
Wireless communication devices, such as mobile telephones, two-way pagers, and the like, receive and transmit information via RF signaling. The pervasiveness of these devices places increasing demands on device manufacturers and wireless service providers to support ever greater numbers of users with increasingly sophisticated voice and data services, all while maintaining a consistently high quality of service.
The advent of digital modulation schemes allows greater numbers of users to be supported within a finite bandwidth. Such schemes also afforded, to varying degrees, opportunities for increased data rates, better data integrity, and enhanced data security. These benefits and others contributed to the success of wireless communication air interface standards such as the Global Services for Mobile Communications (GSM) standard, the Telecommunication Industry Association/Electronic Industry Association (TIA/EIA) standard TIA/EIA-136, and the TIA/EIA interim standard, IS-95.
These existing standards are evolving into so-called third generation (3G) standards, exemplified by the code division multiple access standards cdma2000 and wideband CDMA (W-CDMA). Other developing standards include the Evolution through Enhanced Data Rates (EDGE) extensions to GSM. These existing air interface standards are based on digital modulation schemes of varying sophistication. However, there exists a continuing trend toward requiring greater bandwidth efficiency and more sophisticated signal modulation techniques. Such modulation techniques include 8-PSK and 16-QAM, which are, respectively, phase shift keying (PSK) and quadrature amplitude modulation (QAM) techniques.
The present invention provides a system and method for efficient, linear RF signal amplification over a range of transmit signal power. A branched power amplifier circuit comprises two or more power amplifier branches that may be selectively enabled in varying combinations based on, for example, required transmit signal power levels. Each power amplifier branch comprises a power amplifier biased for saturated mode operation. Each power amplifier, when enabled, provides efficient amplification for the RF input signal provided to the branched power amplifier. Typically, the RF input signal is a constant envelope signal. To impart desired amplitude modulation to the output signals from the power amplifiers, an amplitude modulation circuit powers selected power amplifier branches, while modulating either the supply voltage or supply current supplied to the branches.
Generally, the amplitude modulation circuit includes selection logic that enables or disables individual selected lossy modulators based on the branch selection signal. The lossy modulators, when enabled, provide power to corresponding ones of the power amplifier branches. Depending upon the implementation of the power amplifier branches, the lossy modulators are configured as variable resistive loads or controllable current sources. As a variable resistance, the lossy modulator controls the supply voltage provided to a supply input of a corresponding power amplifier branch responsive to an amplitude information signal. When configured as a controllable current source, each lossy modulator sets the supply current of its corresponding power amplifier branch based on the amplitude information signal.
The branched power amplifier may be configured so that the output power of individual branches, or selected combinations of the individual branches, is set at the point of peak operating efficiency for that branch or combination of branches. The output powers available from the branched power amplifier may be matched to the power control steps defined for one or more air interface standards. For example, IS-136 defines 4 dB steps in transmit signal power. In this sense, the branched power amplifier operates as a composite power amplifier having an xe2x80x9ceffectivexe2x80x9d device size that can be dynamically adjusted based on needed transmit signal power.